The X-ray CT apparatus is an apparatus which emits X-rays as fan beams or cone beams (conical or pyramid shaped beams) to the subject, acquires projection data which is the information regarding the intensity of the X-rays transmitted through the subject, and forms the distribution information of an X-ray absorption coefficient inside the subject as an image on the basis of the projection data.
When the X-ray tube current is constant as a scanning condition of the X-ray CT apparatus, the amount of X-rays may be excessive or deficient depending on the emission angle or an emitted portion of X-rays. In this regard, an X-ray CT apparatus having a tube current automatic control function (CT-AEC (Automatic Exposure Control; the same hereinbelow)) of calculating an elliptical cross-sectional model of the subject from the scanogram projection data and calculating the X-ray tube current value from the area and the aspect ratio of the elliptical cross section is known as a technique in the related art (for example, refer to PTL 1). Using this technique, the amount of X-rays can be optimally controlled so that the target image quality set in advance can be realized. Accordingly, it is possible to maintain the almost constant image quality irrespective of an emitted portion.
On the other hand, the subject has a shape in which the thickness at the body axis center is large and the periphery is thin. Accordingly, X-rays emitted from the X-ray tube are transmitted through a Bow-tie filter (generally, called a Bow-tie filter or a compensation filter; hereinafter, called a compensation filter) in which the thickness at the center (position on a line connecting a focal point of X-rays generated by an X-ray tube and the gantry center) is small and the thickness of the periphery is large. As a result, the intensity of X-rays transmitted through the subject is adjusted so as to be almost constant in a body width direction. That is, by emitting X-rays transmitted through the thinnest portion of the compensation filter to the center of the subject having an almost elliptical cross section, the intensity of X-rays transmitted through the subject is adjusted so as to be almost constant in the body width direction.